1. Field of the Invention
This invention relates to the field of electroceramics and more particularly to a sinter-reactive lanthanum chromite ceramic especially suited for use in a solid oxide fuel cell.
2. Description of the Prior Art
Solid oxide fuel cells (SOFC) have high potential in producing electrical energy from cheap fuels or byproduct waste gas streams of the petrochemical and metallurgical industries. The potential of these fuel cells lies in the high efficiency of converting chemical to electrical energy and could find extensive applications in the domestic, commercial, defense, and aerospace sectors of the economy. The realization of this potential is contingent on the development of reliable and cost efficient methods of cell fabrication.
One of the solid oxide fuel cell designs resembles a heat exchanger with a honeycomb structure in which the electroactive ceramic components also serve as the structural members and eliminate the need for inert supports. This design is referred to as the monolithic solid oxide fuel cell (MSOFC). The honeycomb structure of the MSOFC is made up of thin layers of four components: (1) anode, usually made of a nickel-zirconia cermet; (2) electrolyte, made of a fully stabilized (cubic) zirconia; (3) cathode, made of strontium-doped lanthanum manganite (LaMnO.sub.3); and, (4) interconnect, made of doped lanthanum chromite (LaCrO.sub.3). The adjacent anode, electrolyte, and cathode layers make up one cell while the interconnect serves as an internal electrical connection between individual cells.
The monolithic solid oxide fuel cell offers lower material costs, the potential for reduced manufacturing costs, and a higher efficiency over other geometries and designs. However, fabrication of these cells is expected to be more complicated because the individual components in thin sheet format must be formed into multilayer sheets which are then converted into a honeycomb structure and must be cosintered at the same relatively low temperature (about 1400.degree. C.) in air. Of particular importance is the sintering behavior of the interconnect material, that is, lanthanum chromite which must be sintered to closed porosity or at least about 94% of its theoretical density.
Lanthanum chromite is a refractory material with a melting point of 2510.degree. C. which requires very high temperatures and controlled atmospheres, i.e. extremely low partial pressures of oxygen for sintering to near theoretical density. Groupp and Anderson (L. Groupp and H. U. Anderson, J. Am. Ceram. Soc., 59, 449 (1976)) have shown that LaCrO.sub.3 does not sinter in air even at temperatures as high as 1720.degree. C. According to the data reported by these investigators, LaCrO.sub.3 could be sintered to 95.3% TD only at 1740.degree. C. and in an atmosphere of nitrogen having an oxygen partial pressure of 10.sup.-11 atm. The main inhibition to densification appears to be the volatilization of chromium oxides in oxidizing atmospheres. The oxidation and volatilization of lanthanum chromite in oxidizing atmospheres at temperatures higher than 1400.degree. C. has indeed been reported by Meadowcroft and Wimmer (75th Annual Meeting of the Am. Ceram. Soc., Cincinnati, (1973) and D. B. Meadowcroft and J. M. Wimmer, Am. Ceram. Soc. Bull., vol. 58, 6I0 (1979)) and involves the oxidation of Cr(III) to Cr(VI) and formation of fugitive CrO.sub.3 which is a gas at the high temperatures of sintering. Therefore, the preparation of lanthanum chromite powders which sinter to closed porosity at temperatures below 1650.degree. C. so that Cr volatilization is minimized is critical for the development of monolithic solid oxide fuel cell fabrication technology. One method of fabricating lanthanum chromite (LaCrO.sub.3) electrodes is disclosed in U.S. Pat. No. 3,974,108. This patent teaches the preparation of strontium-doped lanthanum chromite, i.e., lanthanum chromite in which lanthanum is partly substituted by strontium, from lanthanum oxide, strontium carbonate and chromic acid by slurry mixing these materials, drying the slurry and then firing the dried powder mixture in air at temperatures in the preferred range of 1200.degree. to 1500.degree. C. The resultant fired powder is strontium-doped lanthanum chromite which sinters only at temperatures in excess of 1700.degree. C.
An alternative approach is to use sol-gel technology to prepare high surface area, i.e., very fine grain, and sinter reactive LaCrO.sub.3 powders, compacts of which sinter to full density at temperatures lower than 1700.degree. C. Reduction in sintering temperature is achieved by controlling the composition, homogeneity, grain size, and morphology of the powder. This control is brought about by solution chemistry and improved powder separation and processing technology. One such method for preparing lanthanum chromite has been disclosed by C. N. Rao et al. "Synthesis of Complex Metal Oxides Using Hydroxide, Cyanide and Nitrate Solid Solution Precursors", J. Solid State Chem., vol. 58, 29-37 (1985). This method consists of coprecipitation of lanthanum and chromium hydroxides which are intimately mixed and essentially constitute a solid solution of LaCr(OH).sub.6. This hydroxide solid solution is converted to LaCrO.sub.3 by calcination at 850.degree. C. for 12 hours.
Specifically, Rao et al. teach the coprecipitation of LaCr(OH).sub.6 by adding an aqueous nitrate solution of metal ions to a sodium hydroxide solution with subsequent extensive washing of the resultant hydroxide gel to remove sodium ions. Removal of sodium ions from the gel is required because even a very low concentration of sodium ion markedly changes the properties of the gel and degrades the properties of the resultant lanthanum chromite powder. Moreover, Rao et al. state that ammonium hydroxide base could not be used to coprecipitate a hydroxide containing a divalent metal such as magnesium or strontium which are frequently used as dopants of lanthanum chromite.
Another sol-gel method has been disclosed by U.S. Pat. No. 4,830,780, to Olson et al., for the preparation of lanthanum chromite doped with the divalent ions of magnesium, strontium, calcium or barium by coprecipitation from salt solutions of lanthanum, chromium and dopant ions with ammonium hydroxide. In this patent disclosure, extensive washing of the precipitated gel is allegedly not needed because residual ammonium ion is removed via the gas phase during powder calcination. Upon calcination at temperatures of about 600.degree. C., the gel converts to a single compound with the huttonite structure, LaCrO.sub.4, which upon further calcination at 900.degree. C. converts to pure lanthanum chromite, LaCrO.sub.3, with average particle size of about 0.5 .mu.m. The single phase composition of this powder and its fine grain size are in sharp contrast to the powder which is derived by following the teachings of Rao et al. The lanthanum chromite powder prepared according to the teachings of U.S. Pat. No. 4,830,780 could be sintered to 95.7 % theoretical density at 1650.degree. C. for 4 hours in a graphite furnace and to 78% theoretical density at 1600.degree. C. for 2 hours in a furnace with oxygen partial pressure of 10.sup.-10 atmospheres. Densification of this lanthanum chromite to the indicated densities was much better than what was achievable by the powders of the prior art, as for example, stated by Groupp and Anderson. However, this preparation method does not control the material stoichiometry well.
S. Hayashi et al. utilize another approach for lowering the sintering temperature of lanthanum chromite. In J. Mater. Sci. Lett., vol. 7, p. 457 (1988), Hayashi et al. are said to have prepared chromite powders in which chromium was partially substituted by zinc, copper or strontium/zinc. They prepared these zinc- or copper-doped chromite powders by standard ceramic techniques, that is, mixing of component oxide powders, calcination of the powder mixtures, and size reduction. Hayashi et al. formed compacts of the chromite powders by uniaxial pressing under 100 MPa pressure, followed by cold isostatic pressing under 300 MPa pressure. These compacts had a very high green density of about 60% of theoretical and upon firing in air at 1600.degree. C. for 2 hours achieved a density of about 98% of theoretical when the concentration of zinc was 10 or 15 mol % or the copper concentration was 5, 10 or 15 mol %. The sintering of the zinc- or copper-doped chromites, prepared by Hayashi et al., to high density in air, i.e., oxidizing atmosphere, is in sharp contrast to the behavior of strontium- or magnesium-doped chromites which require a reducing atmosphere. However, the sintering temperature of 1600.degree. C. for the powders of Hayashi et al. is much higher than what is needed for the fabrication of the monolithic solid oxide fuel cell.
Densification of lanthanum chromite at much lower temperatures in air has been disclosed in U.S. Pat. No. 4,749,632 to Flandermeyer et al. This was achieved by the incorporation into the lanthanum chromite of a sintering aid, that is, a compound or mixture of compounds which have melting points much lower than 1400.degree. C. For example, lanthanum chromite mixed with 10 w % boric acid powder was formed into a tape and fired at 1377.degree. C. to a density of about 94% of theoretical. Note that boric acid, H.sub.2 BO.sub.3, melts at about 160.degree. C. with simultaneous dehydration to HBO.sub.2, while boron oxide, B.sub.2 O.sub.3, a product of boric acid upon further dehydration, melts at about 460.degree. C. In another example, the sintering aid was made up of 8 w % (Ca,Cr) oxide, which has a eutectic point at about 1022.degree. C., and 6 w % B.sub.2 O.sub.3 and, because of the low melting point of B.sub.2 O.sub.3, the melting point of this sintering aid mixture would be expected to be very much lower than 1000.degree. C. A mixture of lanthanum chromite with the [B.sub.2 O.sub.3, (Ca,Cr) oxide] sintering aid was fired to about 90% of theoretical density at 1277.degree. C.
Thus, U.S. Pat. No. 4,749,632 teaches sintering of lanthanum chromite at low temperatures in air by the incorporation of relatively large quantities of compounds which melt at low temperatures and are referred to as sintering aids. However, the use of relatively large quantities of low-temperature melting compounds results in migration of some of the sintering aid ions into the adjacent layers during sintering thereby adversely affecting the sintering behavior and electrical performance of these layers. The large amounts of these sintering aids are deleterious to the fabrication and operational performance of the monolithic solid oxide fuel cell.
The need remains in the art for lanthanum chromite materials which sinter at lower temperatures (i.e. close to 1400.degree. C.) and which are especially suited for use in the monolithic solid oxide fuel cell.